In the "subs" tab, you have the option to ignore unchanged layer 2 subscriptions. This document explains why.
re-frame
makes a distinction
between layer 2
and layer 3
subscriptions:
layer 2
subscriptions extract data directly from app-db
. They should be
trivial in nature, which is to say there should be no computation in them beyond
what is necessary to extract a value from app-db
layer 3
subscriptions never take values from app-db
. Instead, they have
layer 2
nodes as inputs (or other layer 3
), and they do the more serious CPU work of computing
a materialised view of those values.We never want to run a layer 3
subscriptions unless it is necessary, whereas layer 2
subscriptions are so trivial that we don't mind running them unnecessarily.
This structure delivers efficiency. You see, all (currently instantiated) layer 2
subscriptions
will run every time app-db
changes in any way. All of them. Every time.
And app-db
changes on almost every event, so we want them to be computationally
trivial.
If the value of a layer 2
subscription tests =
to its previous value, then the further
propagation of values through the signal graph will be pruned.
The more computationally intensive layer 3
subscriptions, and ultimately
the views, will only recompute if and when there has been a change in their data inputs.
We don't want your app recomputing views only to find that nothing has changed. That would be inefficient.
Because layer 2
subs run on every single modification of app-db
, and because
very often nothing has changed, their trace can be a bit noisy. Yes, it happened,
but it just isn't that interesting.
So re-frame-10x
gives you the option of filtering out trace for
the layer 2
subscriptions where the value "this time" is the same as the
value "last time".
On the other hand, if a layer 2
subscription runs and its value is
different to last time, that's potentially fascinating and you'll want to
be told all about it. :-)
To determine whether a subscription is a layer 2 or layer 3, re-frame-10x looks at the input signals to a subscription. If one of the input signals is app-db then the subscription is a layer 2 sub, otherwise it is a layer 3. If a subscription hasn't run yet, then we can't know if it is a layer 2 or 3.
In almost all cases, a subscription will be created (by (subscribe [:my-sub])
)
and run (by dereferencing the subscription) within the same epoch, providing
the layer level. If you see "Layer ?" this means that a subscription was created
but not used. This may indicate a bug in your application, although there are
cases where this is ok.
In most cases, after a few more epochs, that subscription will have run, and we know it's layer level, and can use it for any subscriptions shown on any future (and past) epochs.
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